Mattress woes and backaches!!! please help!?

I've had a mattress for 4 years and have always had it on a metal frame and box springs, but no extra wooden slats (the mattress company never told me I needed these, but I have a queen size and apparently 5 are recommended). My mattress has sagged slightly from the first week of owning it, but the mattress company wouldn't do anything about it (and still didn't tell me about the slats). Now, 4 years later, my back is really starting to ache 24/7 and the sag is not going away when I rise from the bed. Is it too late to put the slats in? Or is the mattress damaged and I need to purchase a new one and start out right this time? Thank you for your help!